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    <title>后代与子代选择器</title>
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    <div>
        <!--这是div的子代-->
        <span>Span 1. In the div.
          <!--这是div的后代-->
          <span>Span 2. In the span that's in the div.</span>
        </span>
        <span>Span 1. 这也是子代
          <span>Span 2. 这是后代
        </span>
    </div>
    <span>Span 3. Not in a div at all</span> 
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